pert:
The problem is that Arduino IDE 1.8.0 and 1.8.1 fail to verify a sketch if the absolute path to it is not specified, even though you are running the command from the sketch folder. You should be able to solve the problem by upgrading to Arduino IDE 1.8.3. If for some reason you need to continue to use 1.8.0 then you will need to specify the path to the sketch.
Ah, that solved the problem. Thank you!